This invention relates to graded index type plastic optical fibers which can be used as optical communication media.
Graded index type plastic optical fibers (hereinafter referred to as xe2x80x9cGI type POFSxe2x80x9d) having a radial refractive index distribution in which the refractive index decreases gradually from the center toward the outer periphery of the optical fiber have a wider frequency bandwidth than step index type optical fibers, and are hence expected to be useful as optical communication media.
In the case of GI type POFs, one having a large numerical aperture (NA) and as small a transmission loss as possible needs to be formed for the purpose of improving its bending loss and its coupling loss with the light source. In order to increase NA, GI type POFs must be designed so that the maximum difference in refractive index (xcex94nd) between the center and the outer periphery of the optical fiber is sufficiently large.
Various methods of making such GI type POFs are known. They include, for example, (1) a method which comprises providing two monomers having different reactivity ratios and yielding homopolymers with different refractive indices, placing these monomers in a cylindrical vessel made of a polymer of these monomers so as to cause the polymer to be dissolved and swollen, polymerizing the monomers, and then drawing the resulting product (JP-A 61-130904); (2) a method which comprises preparing a plurality of polymer blends by using two polymer having different refractive indices at various mixing ratios, spinning these polymer blends to form a multilayer fiber, and then heat-treating this fiber to effect interdiffusion between adjacent layers (JP-A 1-265208); and (3) a method which comprises winding films formed of a plurality of binary copolymers having different copolymerization ratios, and drawing the resulting laminate under heated conditions (JP-B 55-15684).
Moreover, in order to minimize a transmission loss caused by exposure to a thermal history, there is known (4) a step index type optical fiber in which a matching layer showing stepwise changes in refractive index is disposed between the core layer and the cladding layer (JP-A 5-232337). Furthermore, there are known step index type optical fibers in which resins having different refractive indices are laminated to create a stepwise refractive index distribution (JP-A 9-133818 and JP-A 9-133819).
The GI type POFs made by the above-described methods (1) and (2) have the disadvantage that, since all layers are formed of polymer blends, a nonuniform structure due to microscopic phase separation tend to be produced in these POFs and these POFs hence show a great light scattering loss. On the other hand, the GI type POFs made by the method (3) and consisting of styrene-methyl methacrylate copolymers or the like have a great light scattering loss, because the difference in refractive index between the copolymers constituting adjacent layers of the multilayer fiber is too large (e.g., 0.02).
In the method (3), a suggestion is also made about POFs made by winding films formed of binary copolymers of vinyl chloride [Tg (the glass transition temperature of its polymer)=77xc2x0 C.) and vinyl acetate (Tg=27xc2x0 C.), or films formed of binary copolymers of ethylene (Tg=xe2x88x9223xc2x0 C.) and vinyl acetate or ethyl methacrylate (Tg=65xc2x0 C.) or vinyl chloride. However, if it is attempted to form such POFs having a large NA, some layers will have a low glass transition temperature. Consequently, irregularities of the diameter of fiber and the layer structure may be produced during shaping, or irregularities of the layer structure may be produced owing to strains or stresses caused by bending, twisting and other deformation applied during handling after spinning, resulting in an increase in the transmission loss of the POF. Moreover, the POFs will show a marked reduction in resistance to moist heat which is a performance characteristic required from a practical point of view, resulting in an increased transmission loss.
Furthermore, as to the step index type optical fibers in (4), the POF made by using a copolymer of benzyl methacrylate (Tg=54xc2x0 C.) and methyl methacrylate (Tg=112xc2x0 C.) as disclosed in JP-A 5-232337 is such that the proportion of the monomers is 10:1 around the center of the POF and the Tg of the copolymer layer is 60xc2x0 C. or below. For the above-described reasons, the transmission loss of this POF is as great as 680 dB/km and, moreover, this POF has poor practical performance from the viewpoint of resistance to moist heat. Furthermore, the optical fibers disclosed in JP-A 9-133818 and JP-A 9-133819 also have similar problems because in JP-A 9-133818 is used a homopolymer of benzyl methacrylate around the center and in JP-A 9-133819 is used a homopolymer of 2,2,2-trifluoroethyl methacrylate (Tg=75xc2x0 C.) around the outer periphery.
An object of the present invention is to provide a wide-band width POF having a small transmission loss, a relatively large numerical aperture, and excellent resistance to moist heat.
The above object is accomplished by a graded index type optical fiber having a multilayer structure comprising a plurality of concentrically arranged non-blended layers formed of (co)polymers which have a glass transition temperature (Tg) of 80xc2x0 C. or above and are selected from the group consisting of two or more homopolymers HP1, HP2, . . . and HPn (in which n is an integer of 2 or greater) composed of units of vinyl monomers M1, M2, . . . and Mn, respectively (provided that the refractive indices of the homopolymers decrease in that order), and one or more binary copolymers CPs composed of units of vinyl monomers M1, M2, . . . and Mn, wherein the refractive index is highest at the center of the multilayer structure and decreases gradually toward the outer periphery thereof.